1. Field of the Invention
The present invention relates to oil seals for establishing a seal between relatively movable shaft and housing. More particularly, the present invention is concerned with lip-type oil seals having hydrodynamic pumping means and a method of making thereof.
2. Description of the Prior Art
Lip-type oil seals having hydrodynamic pumping elements are disclosed, for example, in U.S. Pat. No. 3,347,554 to Jagger et al and U.S. Pat. No. 3,534,969 to Weinand. The seal includes a sealing lip defined by a pair of frustoconical surfaces. Under static conditions of the seal, the sealing lip resiliently engages with the shaft to form a seal band whereby a static fluid seal is achieved.
The frustoconical surface facing the atmospheric region is provided with a plurality of helical pumping ribs or screw threads. Under dynamic conditions, the pumping ribs scrape oil film on the shaft and hydrodynamically feed oil back to the sealed region. In certain conventional designs, the pumping ribs have a uniform triangular cross-section throughout the length thereof.
The problem associated with the conventional pumping ribs having a uniform triangular cross-section is that the hydrodynamic pumping capacity thereof is prematurely decreased with increasing wear.
U.S. Pat. No. 4,094,519 to Heyn et al discloses an oil seal having so-called "bilge-shaped" pumping ribs. Each of the pumping ribs is defined by a pair of curved side faces meeting with each other to define a curved working edge. The pumping ribs have a varying cross-section but have no height at the end near the static seal band.
The advantage of the "bilge-shaped" pumping ribs over the pumping ribs of the uniform triangular cross-section is that a satisfactory pumping action is achieved even after a substantial wear has taken place.
However, the problem which must be overcome in manufacturing the oil seals having the bilge-shaped pumping ribs is that it is extremely difficult to trim the sealing lip with a complete accuracy. Generally, trimming of the seal lips is carried out by a trimming machine wherein a trimming knife is moved along a trim line by a hydraulic cylinder. Unfortunately, the positioning of the trimming knife could not be perfectly accurate and generally involves a tolerance of about 0.2-0.3 mm.
If unfortunately the trim line were offset outwardly of the very end of the bilge-shaped pumping ribs, the pumping ribs of a substantial height would appear on the sealing edge. This causes lifting of the sealing edge away from the shaft and results in a failure of initial static sealing.
If conversely the trimming knife were inadvertently offset inwardly of the end of the bilge-shaped pumping ribs, the pumping ribs would not be brought into contact with the shaft for a sufficient extent. The result of this is that the initial hydrodynamic pumping action is insufficient.